As is conventionally known, in a diesel engine used as an automobile engine or the like, fuel injection control is performed that adjusts a fuel injection timing and a fuel injection amount from a fuel injection valve (also referred to below as an injector) according to an operating state, such as the engine revolutions, amount of accelerator operation, coolant temperature, and intake air temperature (see below Patent Citations 1 and 2).
Incidentally, diesel engine combustion is composed of premixed combustion and diffusive combustion. When fuel injection from a fuel injection valve begins, first a combustible mixture is produced by vaporization and diffusion of fuel (ignition delay period). Next, this combustible mixture self-ignites at about the same time at numerous places in a combustion chamber, and combustion rapidly progresses (premixed combustion). Further, fuel injection into the combustion chamber is continued, so that combustion is continuously performed (diffusive combustion). Afterward, unburned fuel exists even after fuel injection has ended, so heat continues to be generated for some period of time (afterbuming period).
Also, in a diesel engine, as the ignition delay period grows longer, or as the vaporization of fuel during the ignition delay period grows more intense, a flame propagation speed after ignition will increase. When this flame propagation speed is large, the amount of fuel that burns at once becomes too great, pressure inside the cylinder drastically increases, and so vibration or noise occurs. Such a phenomenon is called diesel knocking, and often occurs particularly when operating with a low load. Also, in this sort of situation, a drastic elevation in burn temperature is accompanied by an increase in the amount of nitrogen oxide (referred to below as “NOx”) produced, and thus exhaust emissions become worse.
Consequently, various fuel injection control apparatuses have been developed in order to prevent this sort of diesel knocking and reduce the amount of NOx that is produced. For example, ordinarily fuel injection from a fuel injection valve is intermittently performed by dividing this injection into a plurality of instances.
Also, Patent Citation 1 discloses performing, as pilot injection in which fuel injection is performed prior to main injection, pilot injection for forming a premix gas in a cylinder, and pilot injection for forming an ignition source for igniting and burning fuel from main injection.
Furthermore, Patent Citation 2 discloses executing main injection by dividing main injection into a plurality of instances, and when the internal combustion engine is under a high load, delaying a start timing of injection of a later stage such that combustion from main injection is interrupted, thus suppressing the production of smoke.    Patent Citation 1: JP 2001-254645A    Patent Citation 2: JP 2002-155791A